Which of the following statement(s) is/are true about an acid?

All of these

The correct answer is option 4. All of these.

Acids are a class of chemical compounds that possess certain characteristic properties. Here's an explanation of each statement:

1. An acid releases \( \text{H}^+ \) ions in aqueous solution: Acids are substances that ionize or dissociate in water to produce hydrogen ions (\( \text{H}^+ \)). This process is often represented by the general equation:
\[ \text{Acid} \rightarrow \text{H}^+ + \text{Anion} \]
The release of \( \text{H}^+ \) ions in solution is what gives acids their characteristic properties.

2. Acids are sour in taste:  Acids have a characteristic sour taste. For example, citric acid in lemons and acetic acid in vinegar are responsible for their sour taste. However, it's important to note that tasting acids can be dangerous and is not recommended due to their corrosive nature.

3. Acid turns blue litmus paper red:  Litmus paper is a common acid-base indicator that changes color in the presence of acids or bases. Blue litmus paper turns red in acidic solutions due to the reaction between the acid and the dye in the litmus paper. The change in color indicates the presence of an acid.

These properties help in identifying and characterizing acids in various chemical reactions and solutions. They are fundamental to our understanding of acids and their behavior in aqueous solutions.